Conventional methods of directional solidification (DS), such as that known in the art as the Bridgman technique, generally entail the use of silica-bonded alumina shell molds for high-temperature casting processes. The chemical reactivity of certain molten materials can seriously degrade ceramic molds during casting, and can cause contamination of the melt by the formation of oxide inclusions and increased interstitial concentrations. Such reactive materials can also degrade the ceramic crucibles in which the materials are melted prior to casting. The reactivity of such materials, including aluminum, titanium, niobium, etc., is the result of a low free energy of oxide formation. While melting and casting operations with reactive materials are performed in an inert atmosphere or vacuum to avoid reactions with gaseous oxygen, oxygen is generally nonetheless available from the mold or crucible as a result of the presence of less stable oxides in the ceramic mold and crucible materials. Significant degradation of the ceramic mold and crucible and contamination of the melt is even more likely to result when molten materials containing a high concentration of one or more reactive elements are in long-term contact with the mold or crucible, and particularly if such materials have a high melting temperature.
As a solution to the above, cold-wall crucible DS methods have been developed to produce ingots of very high temperature alloys and composites containing reactive elements. Of these, segmented, water-cooled copper crucibles whose contents are heated by induction have found use. As is known in the art, segmentation in the crucible wall enables induction heating to occur through the metal crucible walls by interrupting induced current flow in the walls that would otherwise attenuate the field of the induction coil. A slag can also be used between the melt and crucible walls to prevent reactions from occurring therebetween, as well as to prevent shorting between segments of the crucible walls.
As reported by Chang et al. in Cold-Crucible Directional Solidification of Refractory Metal-Silicide Eutectics, The Journal of the Minerals, Metals & Materials Society (JOM), Vol. 44, No. 6 (June 1992), pg. 59-63, directionally solidified high-temperature eutectic composites of reactive elements have been successfully grown with a segmented, water-cooled copper crucible in a vacuum or inert atmosphere. Induction coils used to heat the melt also serve to induce melt convection to promote homogeneous mixing, as well as levitate the melt away from the walls of the crucible. Levitation of the melt reduces the contact area between the melt and crucible and, therefore, reduces heat transfer and power loss to the crucible during the melting operation. Chang et al. then effected directional solidification by attaching a seed crystal to a water-cooled pulling rod, which was lowered into the melt. Withdrawal of the seed from the melt and crystal growth from the seed were controlled to achieve directional solidification, and rotation of the pulling rod during withdrawal was employed to maintain the symmetry of thermal conditions during directional solidification.
While the DS technique reported by Chang et al. has been successfully applied to many different alloy systems, yielding DS ingots with much less contamination than those processed through conventional mold-based DS methods, the technique is ultimately limited by the amount of material that can be processed in the crucible, with a maximum diameter being about ten millimeters for ingots produced by the technique. Furthermore, reactions between a melt and the crucible and a melt and the mold, and therefore degradation of the crucible and mold and contamination of the melt, are more likely with increasing reactive element content of the melt. Therefore, further improvements in directional solidification methods would be desirable, particularly in the production of directionally solidified high temperature materials containing reactive elements.